The present invention relates to a knock detecting apparatus for an internal combustion engine, which apparatus has a function for detecting the background level of output signals from a vibration waveform detection section that detects knocking vibration of the internal combustion engine. The present invention also relates to a knock detecting method.
Typically, for detecting knock of an internal combustion engine, knocking vibration of the cylinder block is detected, and the signal level in a knock frequency band from output signals of the knock sensor is extracted at a filter. Then, the signal level in the knock frequency band is compared with a knock determining level. Knock determination is thus performed. In this case, the knock determining level is set with reference to the background level of the output signals of the knock sensor. Therefore, the background level needs to be detected accurately.
The background level changes according to the operating state of the internal combustion engine. For example, in Japanese Examined Utility Model Publication No. 6-41151, output signals of a knock sensor are subject to smoothing process at every ignition during the operation of the internal combustion engine to obtain the background level. However, in a transient operating state in which the signal level of the knock sensor is abruptly changed, a response delay is caused in the obtained background level, which can lead to an erroneous detection of knock.
To deal with this drawback, Japanese Laid-Open Patent Publication No. 2002-47993 discloses a technique, in which, during a knock determination period set for the combustion stroke of each cylinder, a signal level in a frequency band that is different from the knock frequency band is extracted from an output signal of a knock sensor (hereinafter, the different band will be referred to as a background frequency band), and the background level is computed, accordingly.
Further, U.S. Pat. No. 6,246,953 discloses another technique, in which a background level is computed based on an output signal of a knock sensor during a period that is different from the knock determination period (hereinafter, the different period will be referred to as a background determination period).
However, when knock occurs, the signal level in the knock frequency band is significantly increased. Accordingly, the signal level in the background frequency band, which is different from the knock frequency band, is increased. Therefore, if the background level is detected based on the signal level in the background frequency band as in Japanese Laid-Open Patent Publication 2002-47993, the background level is detected to be greater than the actual value when knock occurs. Further, when noise is superimposed on the background frequency band, the background level is detected to be greater than the actual value.
Also, in recent years, to increase power, improve fuel economy, and reduce emission, more and more engines are equipped with a variable valve actuation mechanism such as a variable valve timing mechanism or with a system for varying fuel injection timing of in-cylinder fuel injection. In these internal combustion engines, a period in which contacting noise of intake and exhaust valves and actuation noise of the fuel injection valves are contained in output signals of the knock sensor is changed according to the control state of the engine. Therefore, it is becoming difficult to uniformly set a background determination period in which such noise is not produced. Thus, the method disclosed in U.S. Pat. No. 6,246,953 has a drawback shown below. That is, although depending on the control state of an internal combustion engine, contacting noise of intake and exhaust valves and actuation noise of the fuel injection valves are superimposed on output signals of a knock sensor in a background determination period (see FIG. 6). This causes the background level to be detected as a value greater than the actual value.